1. Field of the Disclosure
This invention relates generally to logging tool apparatus and techniques for logging wells. More specifically, the invention relates to antenna assemblies for nuclear magnetic resonance (NMR) well logging apparatus.
2. Background of the Related Art
To obtain hydrocarbons such as oil and gas, a drilling assembly (also referred to as the “bottom hole assembly” or the “BHA”) carrying a drill bit at its bottom end is conveyed into the wellbore or borehole. Bottom hole assemblies (“BHA”) generally include several formation evaluation sensors for determining various parameters of the formation surrounding the BHA during the drilling of the wellbore. Such sensors are usually referred to as the measurement-while-drilling (“MWD”) sensors. Such sensors traditionally have electromagnetic propagation sensors for measuring the resistivity, dielectric constant, or water saturation of the formation, nuclear magnetic resonance (“NMR”) sensors for determining the porosity of the formation and acoustic sensors to determine the formation acoustic velocity and porosity.
NMR sensors utilize permanent magnets to generate a static magnetic field in the formation surrounding the MWD tool. A radio frequency (RF) coil disposed between the magnets or around the magnets induces a RF magnetic field. The magnets and the RF coils are positioned so that the static and RF fields are perpendicular to each other at least over a portion of the formation surrounding the NMR tool. The NMR measurements corresponding to the formation are used to determine the formation parameters of interest.
The antenna assembly includes parts which are exposed to the wellbore pressure. Exposure to high pressures, high temperatures, and corrosive fluids may cause degradation of certain antenna assembly parts. For example, in wellbores in gas producing formations, rubber and plastic parts of antenna assemblies may degrade over time due to exposure to high pressures, causing fluid to leak into the tool, causing short circuits and damaging electronics and electrical components. In addition, rubber components can suffer from explosive decompression when applied under downhole conditions and retrieved back to surface again. This process may destroy the rubber and can make the tool unusable without repair.